Portable wireless communication devices such as radiotelephones sometimes use one or more antennas to transmit and receive radio frequency signals. In a radiotelephone using two antennas, the second antenna should have comparable performance with respect to the first, or main, antenna and should also have sufficient decorrelation with respect to the first antenna so that the performance of the two antennas is not degraded when both antennas are operating. Antenna performance is a combination of many parameters. A sufficient operating frequency bandwidth, a high radiation efficiency, and a desirable radiation pattern characteristic, and a low correlation, are all desired components of antenna performance. Correlation is computed as the normalized covariance of the radiation patterns of the two antennas. Due to the dimensions and generally-accepted placement of a main antenna along the major axis of a device such as a hand-held radiotelephone, however, efficiency and decorrelation goals are extremely difficult to achieve.
FIG. 1 shows a prior art two-antenna structure implemented in a hand-held radiotelephone 130. A first antenna 140 is a retractable linear antenna. When the first antenna is fully-extended, as shown, the length of the first antenna is a quarter wavelength of the frequency of interest. Note that the first antenna 140 is aligned parallel to the major axis 145 of the radiotelephone 130 and has a vertical polarization with respect to the ground 190.
The radiotelephone 130 also has a microstrip patch antenna as a second antenna 150 attached to a printed circuit board inside the radiotelephone 130 and aligned parallel to a minor axis 155 of the radiotelephone 130 to send or receive signals having a horizontal polarization with respect to the ground 190. In isolation, the second antenna 150 may well produce horizontally polarized signals, but when the second antenna 150 is attached to the printed circuit board and in the proximity of the first antenna 140, the polarization of the second antenna 150 reorients along the major axis 145 of the radiotelephone 130. As the polarization of the second antenna reorients, the first antenna 140 and second antenna 150 become highly correlated and many of the advantages of the two-antenna structure are lost. Commonly, a prior art two-antenna structure implemented in a radiotelephone has a correlation factor of over 0.8 between the two antennas. Effective diversity operation requires a correlation factor of less than 0.6 between the two antennas.
The reorientation of the polarization of the signals from the second antenna 150 is due to various factors, including the fact that hand-held radiotelephones typically have major axis 145 and minor axis 155 dimensions with an aspect ratio greater than 2:1 and that the major dimension of the radiotelephone is significant with respect to the wavelength of operation while the other dimensions of the radiotelephone are small with respect to this wavelength. Additionally, because the minor dimension of the radiotelephone is small with respect to the wavelength of interest, the second antenna 150 is easily perturbed and detuned, which creates susceptibility to effects of the hand or head of a user 110 on antenna efficiency.
Thus there is a need for a two-antenna structure that maintains decorrelation and efficiency between a first antenna aligned along a major axis of a portable wireless communication device and a second antenna.